Dust clouds around red giant stars: Evidence of sublimating comet disks?

Abstract

A growing amount of observational and theoretical evidence suggests that most main sequence stars are surrounded by disks of cometary material within several hundred astronomical units. In this paper we investigate dust production by comets in such disks when the central stars evolve up the red giant and the asymptotic giant branch (AGB) where their luminosities can increase to 10 4−10 5 L ⊙. Once released, the dust will undergo collisions, ablation, and be accelerated by the gas outflow, whereupon the smaller fragments (≳10 a ̊ ) can become the cores necessary for condensation of the gas. The origin of the requisite cores has presented a well-known problem for classical condensation theory. This explanation is consistent with the dust production observed around red giants and AGB supergiants (which have increasing luminosities) and the fact that earlier supergiants and most WR stars (whose luminosities are unchanging) do not have significant dust clouds even though they have significant stellar winds. Another consequence of the model is that the dust condensation radius is greater than that predicted by conventional theory, in agreement with IR interferometry measurements of α-Ori. A further prediction is that the spatial distribution of the dust (i.e., disk-like) will not, in general, coincide with that of the gas outflow, in contrast to classical condensation theory. The latter test is relevant to the dust shells that condense in classical novae outbursts, which we also interpret in terms of disks of cometary material. Verification of these interpretations would provide new evidence that circumstellar disks of cometary material may be ubiquitous.